In electronic image reproduction it is well known to mount a picture original on a rotary drum and helically scan the original with a light source in a scanning head which traverses the drum. The reflected light from the original is conducted by way of different color filters to photoconductors whose electrical outputs thereupon represent the tonal densities of the measured colors in each pixel comprising the original. These electrical outputs are digitized and then used to control an exposing scanner or plotter whose drum carries a recording medium such as a light-sensitive film which is scanned and exposed in accordance with the aforesaid picture signals.
Color images for pictures are typically reproduced in half-tone copy through the use of a separate black-and-white half-tone film or separation for each color to be reproduced. Each separation carries a pattern of dots which are spaced along a screen line with the dots being of various sizes in accordance with the grey-tone levels for the particular color component of the original to be reproduced.
In the production of such half-tone color separations or films, it is known to scan the picture original and to superimpose on the picture signal screen signals corresponding to a vignetted contact screen. The difference signals are thereupon applied to the plotter which exposes the film. The film has a selected sensitivity threshold so that the size of each exposed dot on the film varies directly with the local picture brightness for that particular color.
As is well known, an interference pattern or Moire effect is caused by the screen line patterns of the half-tone dots when a plurality of such half-tone color images are printed over one another to reproduce the color original. In order to minimize Moire, the grid structures of the respective color separations necessary for the reproduction of a multicolor picture original should be rotated relative to one another by selected angles. Typically in the graphic arts industry, the screen lines for the color black are oriented at 45 degrees, those for cyan at 15 degrees, those for yellow at 0 degrees and those for magenta at 75 degrees (or -15 degrees).
There has already been proposed the dot-by-dot and line-by-line rastered recording of picture signals obtained by scanning a picture original. This includes superimposing the picture signals on raster signals such as are produced by scanning, at a finer resolution than that used for the picture scanning, a screen having an internal structure defined by mutually perpendicular sets of lines rotated at an angle relative to the direction of scanning of the original, which angle has a rational tangent. The raster signals are obtained and stored in a memory. The contents of the memory are then called up in a periodic fashion, picture line by picture line, as the original picture is being recorded and are superimposed on the picture signals. The combined signals are then used to control a plotter which exposes the reproduction medium. An arrangement such as this is shown for example in U.S. Pat. No. 3,657,422.
That technique for making screen separations is disadvantaged, however, in that it requires that there be used screen angles which have rational tangents, rather than the screen angles preferred by the graphic arts industry which do not necessarily have rational tangents. Thus, for example, in order to practice that prior method, one must use odd screen angles such as 14.03 degrees, 18.4 degrees and 22.5 degrees. Such angles are not the optimum angles for achieving the best quality reproduction of color originals. Moreover, in the apparatus described in that patent, the vignetted screen must have a pyramid-type cell configuration, whereas in many printing applications other cell configurations, Respi and split-dot, for example, are more preferable. Finally, prior arrangements of that general type are not as versatile as they might be in their ability to change the shape and size of the screen cells for different reproduction applications.